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The phase competition between nematic order and superconductivity has been intensively investigated in iron-based superconductors. Recent neutron pair-distribution-function (PDF) measurements on polycrystaline samples provide evidence of dynamic orthorhombic distortions, even within the C₄ tetragonal magnetic phase of hole-doped compounds [1]. We present synchrotron x-ray measurements of diffuse scattering from single crystals of (Ba,Na)Fe₂As₂, which show that these nematic fluctuations are predominantly two-dimensional. The measurements cover a sufficiently large volume of reciprocal space that they can be transformed into real space, generating 3D PDFs, from which both in-plane and out-of-plane correlation lengths are determined as a function of temperature and dopant concentration. These results will be compared to other measurements of the nematic susceptibility.

[1] B. A. Frandsen, et al., Phys Rev Lett 119, 187001 (2017).